1. Field of the Invention
This invention relates to drugs for enhancing learning potential and to methods of their use for improving learning capacity. More particularly, this invention relates to drugs that induce long term potentiation (LTP) and enhance cognitive function in mammals.
2. Description of Related Art
The hippocampal formation, which is a specialized region of the limbic cortex located in the temporal lobe, is also known as the comu ammonis from which come the names of its two major divisions, CA1 and CA3. Neurons in the entorhinal cortex relay incoming information through a bundle of axons called the perforant path to neurons in the dentate gyrus, another region of the hippocampal formation. All association areas of the brain send information to, and receive information from, the hippocampal formation, via the entorhinal cortex. Thus, the hippocampal formation is in a position to know—and to influence—what is going on in the rest of the brain. Several neurotransmitters, including glutamate, gamma-aminobutyric acid (GABA), noradrenalin, serotonin, and acetylcholine have a profound effect on the activity of the hippocampal formation and, undoubtedly, on its functions.
LTP is a use-induced increase in the magnitude of the postsynaptic response (i.e., the strengthening of the synaptic connection between two neurons) caused by binding of glutamate to receptors in the postsynaptic membrane. However, the maintenance of LTP may involve both presynaptic and postsynaptic mechanisms (for review, see Bliss & Collingridge, Nature, 361:31–39, 1993). This increase is understood to signify that LTP is associated with enhanced cognitive function. LTP was first discovered in 1966 to result from a response to high frequency electrical stimulation (tetanus) of the axons in the perforant path with a burst of approximately one hundred pulses of electrical stimulation, delivered within a few seconds. Evidence that LTP has occurred is obtained by periodically delivering single pulses to the perforant path and recording the response in the dentate gyrus. If, for example, the population EPSP is larger than it was before tetanus, LTP has taken place. LTP can also be produced in fields CA3 and CA1 as well as in the neocortex (Perkins and Teyler, Brain Research, 439:25–47, 1988; Brown, et al., in Neural Models of Plasticity: Experimental and Theoretical Approaches, ed. J. H. Byrne and W. O. Berry, San Diego: Academic Press 1989). It can last for several months (Bliss and Lømo, J. Physiol., 232:331, 1973). Even more importantly, LTP can involve the interaction between different synapses on a particular neuron. That is, when weak and strong synapses on a single neuron are stimulated at approximately the same time, the weak synapse becomes strengthened, and this strengthening of the synapse has been proposed to be the basis of long-term learning (Hebb, The Organization of Behavior, New York: Wiley-Interscience, 1949). This phenomenon, produced by the association (in time) between the activity of two synapses, is called associative LTP because it resembles what happens during classical conditioning for learned responses.
Many experiments have demonstrated that associative LTP can take place in hippocampal tissue slices, which are maintained in a temperature-controlled chamber filled with a liquid that resembles interstitial fluid and remain alive for up to 40 hours. For example, Chatterji, Stanton, and Sejnowski (Brain Research, 95:145–150, 1989) stimulated two sets of axons: the axons that connect the dentate gyrus with field CA3 (strong input) and a set of collateral axons arising from other pyramidal cells in CA3 (weak input). The researchers found that when the weak input and the strong input were stimulated together, the response of the CA3 pyramidal cells to the weak input increased. LTP occurs when a sufficient amount of calcium enters the post-synaptic neuron. When a hippocampal slice is placed in a solution that contains very little calcium, LTP does not take place (Dunwiddie and Lynch, Brain Res., 169:103–110, 1979). It was found that induction of LTP could be blocked by the intracellular injection of the calcium chelator (EDTA) (Lynch, et al., Nature, 305:719–721, 1983). These results suggested that calcium signaling is involved in the induction of LTP Several calcium-sensitive enzymes have been proposed to play a role in converting the initial calcium signal into persistent modifications of synaptic strength. For example, entry of calcium into the dendritic spine causes structural changes in the spine by activation of the enzyme calpain, which is instrumental in break-down of spectrin, a protein that appears to serve as a framework support for the structure of the spine. These changes decrease the electrical resistance between the spine and the rest of the dendrite, thus increasing the effect of EPSPs on the dendritic membrane potential. Staubli, et al. (Brain Res., 444:153–158, 1988) found that proteolytic activity of calpain is blocked by infusion of a drug called leupeptin into lateral ventricles of rats with the result that electrical stimulation of the hippocampus no longer produces LTP.
Some sort of additive effect is required to produce LTP. A series of electrical pulses delivered at a high rate all in one burst will produce LTP, but the same number of pulses given at a slow rate will not. Apparently, each pulse produces an aftereffect that dissipates with time. If the next pulse comes before the aftereffect fades away, its own effect will be amplified. Thus, each pulse “primes” the following one. If the interval between pulses is chosen carefully, very little stimulation is required.
Experiments have shown that the priming effect consists of a depolarization of the postsynaptic membrane: the depolarization caused by one pulse primes the synapse for the next one. The N-methyl-D-aspartate (NMDA) receptor, a type of glutamate receptor named for its agonist, are glutamate activated calcium permeable ion channels, which are normally blocked by magnesium ions to prevent calcium ions from entering the cell. When the postsynaptic membrane is depolarized the magnesium ion is ejected from the ion channel, and in the presence of glutamate, the channels will open, allowing calcium ions to enter the dendritic spine where they effect the physical changes responsible for LTP. Collingridge, et al. (J. Physiol., 334:33–46, 1983) discovered that drugs, such as APV (2-amino-5-phosphonopentanoate), that block N-methyl-D-aspartate (NMDA) receptors prevent LTP from taking place in the CA1 field and the dentate gyrus. However, such drugs had no effect on any LTP that had already been established, indicating that the NMDA receptor is not involved in the maintenance of LTP.
While LTP is a strong cellular model for learning and memory, whether LTP participates in memory formation is still unresolved. In general, treatments that affect LTP also have consequences for the acquisition of certain learning tasks. When rats are exposed to novel, complex environments, the extracellular population spike in the dentate gyrus increases, just as it does when the entorhinal cortex is subjected to high-frequency stimulation, and treatments that interfere with LTP also interfere with the learning of tasks in which the hippocampus is involved. For instance Morris, et al. (Nature, 319:774–776, 1986) trained rats in manoeuvreing a “milk maze”, a spatially guided task in separate experiments. When APV was infused into the animals' lateral ventricles, the animals were prevented from learning the task. Physical damage to the hippocampus results in the same inability to learn a spatially guided task. Perhaps the strongest support for a role for LTP in learning and memory will come from the discovery of a drug that induces LTP in vivo and enhances at least one form of learning in mammals.
Heretofore, no drug has been discovered that will chemically stimulate the hippocampus so as to cause LTP in vivo when administered peripherally. Drugs that induce LTP may be effective in the treatment of cognitive impairment such as memory loss and learning dysfunction associated with Alzheimer's Disease, stroke and other neurological disorders. They may also be useful for enhancing learning potential in the healthy mammalian brain. The practical utility of such a drug will depend upon its ability to cross the blood-brain barrier without inducing deleterious effects, such as seizures and convulsions, since injection into the lateral ventricals of the brain is not a practical method of enhancing learning in humans and animals. Brief application in vitro to slices of rat hippocampus of K+ channel blockers (tetraethylammonium, mast-cell-degranulating peptide) or the metabotropic glutamate receptor agonist aminocyclopentane-1S,3R-decarboxylate (1S,3R-ACPD) induces a long lasting potentiation of synaptic responses (Cherubini, et al., Nature, 328:70–73, 1987; Aniksztejn and Ben-Ari, Nature, 349:67–69, 1991; and Bashir, et al., Nature, 363:347–350, 1993). However, when mast-cell-degranulating peptide or 1S,3R-ACPD are administered directly into the brain, both compounds produce seizures and convulsions in experimental animals (Cherubini, et al., Nature, 328:70–73, 1987; Bidard, et al., Brain Res., 48:235–244, 1987; Sacaan and Schoepp, Neurosci. Lett., 139:77–82, 1992). Therefore, the need exists for the discovery of new compounds capable of inducing LTP in vivo and for methods of utilizing them so as to enhance learning ability in humans and animals.